In general, a microwave oven in which food is heated by microwaves shows dewing either on an inside surface of a door and/or on an inside wall of a cooking chamber owing to water vapor in the cooking chamber generated when the food is heated. To prevent this, a fan is used to blow an appropriate amount of dry air into the cooking chamber for discharging the air to outside of the cooking chamber. The microwave oven is provided with a humidity sensor at an air outlet for detecting a humidity of the air for implementing auto cooking by detecting a heated degree of the food according to the humidity. Of the humidity sensors used for the microwave ovens, a humidity sensor of a thermister element is typical one, in which the resistance is varied with temperature.
A humidity sensor of the thermister element and a method for controlling a microwave oven of the humidity sensor will be explained, with reference to the attached drawings. FIG. 1 illustrates a section of a related art thermister type humidity sensor, FIG. 2 illustrates a front and a plan views of the related art thermister type humidity sensor, and FIG. 3 illustrates a circuit of a related art thermister type humidity sensor.
Referring to FIG. 1, the related art thermister type humidity sensor 10 is provided with two spaces formed by a stem 11 and a cap 12, a humidity sensing thermister 13 in one of the spaces, and a temperature compensating thermister 14 in the other space. Each of the thermisters 13 and 14 are connected to lead pins 15 passed through the stem 11 by a platinum wire 16 to form a circuit. There is a detection hole 17 in a top of the cap 12 of the humidity thermister 13 for introduction of water vapor. The thermisters 13 and 14 are of NTC thermisters in which temperature and resistance are inversely proportional.
Referring to FIG. 2, the humidity sensor of an NTC thermister is provided with a front case 1 and a rear case 2 for forming spaces thermisters are to be fitted therein respectively, a heat unit 3 fitted in the front case 1 for accommodating the thermisters 13 and 14 and maintaining a temperature thereof, shielded wires 5 connected to the lead pins for applying a device signal and preventing noise. There are a plurality of fastening holes for fitting the case.
Referring to FIG. 3, the thermister type humidity sensor is provided with the humidity sensing thermister 13, the temperature compensating thermister 14 connected with the humidity sensing thermister 13 in series for compensating a voltage variation caused by a resistance variation of the humidity sensor thermister 13, an amplifier 100 having an inversion terminal (−) for receiving an output voltage of the humidity sensing thermister 13 and a non-inversion terminal (+) for receiving a voltage, for amplifying a difference of the voltages, and a variable resistor VR for providing a voltage variation of the output voltage caused by the resistance variation of the humidity sensing thermister 13 and applying the voltage variation to a non-inversion (+) terminal of the amplifier 100. The foregoing thermister type humidity sensor detects the humidity by using a resistance difference caused by a temperature difference between the humidity sensing thermister 13 and the temperature compensating thermister 14 when water vapor is introduced into the humidity sensing thermister 13 through a detection hole 17 in the stem 11.
A related art method for automatic control of a cooker having the thermister type humidity sensor applied thereto will be explained. FIG. 4 illustrates a flow chart showing the steps of a related art method for controlling a cooker having the thermister type humidity sensor applied thereto.
Referring to FIGS. 3 and 4, when the user selects a cooking setting mode of the cooker for cooking a food to a desired state, the cooker puts the fan (not shown) into operation for a preset blowing time period (S10), and determines whether the blowing is completed (S11). When the blowing is completed as a result of the determination (S11), the magnetron (not shown) is put into operation and the fan blowing is continued (S13), when the humidity sensing thermister senses a variation of the humidity in the cooker generated as the magnetron is put into operation. That is, a resistance of the humidity sensing thermister is varied with the water vapor in the cooker generated as the magnetron is put into operation, and the output voltage is varied with the resistance variation. Then, the output voltage V1 of the humidity sensing thermister is applied to an inversion terminal (−) of the amplifier 100, and perfection of a zero balance is determined (S14). That is, at the time the magnetron is put into operation, the variable resistor VR is varied so that a voltage V2 to the non-inversion terminal (+) of the amplifier 100 is the same with a voltage V1 applied to the inversion terminal (−) of the amplifier 100, for initializing an output voltage of the amplifier 100, i.e., a sensor output value V0. Next, upon perfection of the zero balance (S14), setting of the sensor output value V0 from the amplifier 100 is determined of being an initial value Vref (S15). As a result of the determination (S15), if it is found the sensor output value V0 is set to the initial value Vref, reach to a voltage variation ΔV required for a specific menu with reference to the sensor initial value is determined. That is, after the sensor output V0 is set as the initial value Vref, there is a voltage variation ΔV between the voltage of a menu the user selected and the sensor initial value Vref, and reach of the sensor output V0 to the voltage variation ΔV is determined. As a result of the determination (S16), if the sensor output V0 reaches to the voltage variation ΔV of the menu with reference to the sensor initial value Vref, a time period T1 required to the voltage variation ΔV is calculated, to calculate a main operation time period (S17). Next, the magnetron is operated for the main operation time period T2 period, and fan is blown, to cook the food, and the magnetron and the fan are stopped (S18). If the sensor output V0 is not set as the sensor initial value Vref, pass of a preset operation time period of the magnetron is determined (S19). As a result of the determination (S19), if it is found that the preset operation time period is passed, the sensor initial value Vref is set (S20), after the present voltage variation ΔV reach time period is increased by ‘1’ second (S21), the reach to the voltage variation ΔV of the menu with reference to the sensor initial value Vref is determined (S22). As a result of the determination (S22), if the voltage variation is not reached to the voltage variation ΔV of the menu, the process proceeds to the step (S21) of increasing the voltage variation ΔV reach time period by ‘1’ second. Opposite to this, if the voltage variation is reached to the voltage variation ΔV of the menu, a step (S17) of calculating the main operation time period T2 is carried out by calculating a time period T1 required to reach to the voltage variation ΔV. Meanwhile, as a result of the determination (S16), if it is found that the sensor output V0 is not reached to the voltage variation ΔV of the menu with reference to the sensor initial value Vref, the step (S21) of increasing the present voltage variation ΔV reach time period by “1” second is carried out until the voltage variation reaches to the required voltage variation of the menu. At the end, a humidity change in the cooker is expressed as the sensor output V0 of the thermister type humidity sensor.
FIG. 5 illustrates a graph showing characteristics of a related art thermister type humidity sensor, wherefrom it can be known that a negative temperature coefficient thermister element has a non-linear resistance variation to a temperature change. That is, the negative temperature coefficient thermister element has a non-linear-inversely proportional relation in which a resistance is decreased if a temperature is increased, thereby showing difficulty in predicting a temperature to a humidity change since the sensor output is not linear. Because of this, in a case when the related art thermister type humidity sensor is fitted to the air outlet of the cooker and detects humidity in the cooking chamber, accurate detection of the humidity is not possible. At the end, the microcomputer can not know an exact degree of cooking of the food, and, accordingly, can not control output of the magnetron and operation of the fan, precisely. Particularly, if a function to maintain a temperature of the food constant is selected, such a disadvantage becomes so distinctive that the user can not maintain a temperature of the food constant properly. Moreover, in the case of cooker having the related art thermister type humidity sensor applied thereto, control of a cooking time period is not possible if the food wrapped, since the cooker is not provided with any counter measure for a case of wrapped food.
In the meantime, in order to maintain a thermal equilibrium between the related art thermister type humidity sensors, the cap and the stem are provided for enclosing the elements, the elements are placed in a heater unit, and the heater unit is fixed to a case by welding. When the humidity sensor is fitted to a bracket or the like of the cooker, care should be taken so that a good thermal contact is made between the case and the bracket. At the end, above condition makes a fabrication process complicated, and fitting the humidity sensor to the cooker or the like difficult.